Image conversion by a semiconductor junction array



United States Patent lnventor Appl. No.

Filed Patented Assignee Joseph W.C. Harpster Columbus, Ohio Dec. 8, 1967Dec. 29, 1970 The Ohio State University Columbus, Ohio an Institution ofHigher Learning IMAGE CONVERSION BY A SEMICONDUCTOR PrimaryExaminer-Rodney D. Bennett, .lr. Assistant Examiner-Jeffrey P. MorrisAttorney-Anthony D. Cennamo ABSTRACT: The invention is for an imageconversion or in- JUNCTION ARRAY 15 Claims, 5 Drawing Figs.

U.S. Cl. 315/12, 250/21 1; 313/66 Int. Cl. H01j 29/41 Field of Search313/65, 66, 94, 95, 96; 250/21 1, 211.1, 209, 214PHOTO, 214; l78/7.6,7.7; 315/10, 11

SILICON PHOI'OVOLTAGEVOG) VI o-h GENERATION RATE e-h summon an:(Mm/"1300) tensification device utilizing the photovoltaiccharacteristics of semiconductor junctions and an electron beam scan ofthe radiation induced voltage.

IMAGE CONVERSION BY A SEMICONDUCTOR JUNCTION ARRAY BACKGROUNDSolid-state devices have found applications in image panels in a varietyof ways in the prior art. However, it has been found that these priorsystems or devices have certain limitations. Transistors have been usedin display systems without a vidicon or orthicon readout arrangement,but this necessitates the use of extensive complex and expensiveswitching circuitry. When transistors and diodes are used in this mannerthey require a connection to each discrete junction and there results acorrespondingly poor resolution. Reverse bias junction arrays in ,matrixarrangements generally produce poor contrast because of the nonuniformreverse leakage currents. The solid-state image tube disclosed in F. W.Reynolds, US. Pat. No. 3,01 1,089, granted Nov. 28, 1961, utilizes thecapacitive effect of the P-N junction in a standard vidicon tube. TheReynolds panel, in addition to requiring a means for reverse biasing thejunctions, lacks dynamic range because the capacitive variation is muchless than the voltage variation for a large amount of radiationexcitation. A further limitation of these discrete element semiconductortargets is their restriction to imaging with optical radiation.

SUMMARY OF INVENTION The invention relates to image conversion orintensification by an electron beam tube consisting of a mosaic array ofsemiconductor junction devices as the target plate and the otherelements associated with conventional television image tubes. Theinvention utilizes the photovoltaic characteristics of thesemiconductive junction and senses the amount of voltage produced ateach individual junction in the array thereby obtaining the imagepattern which strikes the target plate. Imaging may be by direct orindirect interaction with the tubes target plate, depending upon thedepth of penetration of the incident radiation. Indirect interaction isprovided by conversion of the radiation to electrons or other ionizingparticles which then interact with the mosaic array.

The present invention solves several problems previously existent in theprior art. Reverse biasing of the junctions is not required in theinvention, therefore, simplicity of electrical connection, excellentcontrast, and high resolution is achieved. Transistor devices arecurrent sensitive and bias dependent, whereas the present inventionutilizes the photovoltaic characteristics of semiconductor junctions andis therefore voltage sensitive and not bias dependent. An image panelthat is voltage sensitive permits better contrast, wide dynamic rangeand sensitivity at lower excitation levels than that obtained in priorart systems employing only the capacitive characteristics ofsemiconductor junctions. An embodiment of the invention is shown whichpermits imaging by neutron radiation which was unknown to the prior art.The invention can be applied successfully in many fields,'nondestructivetest ing and radiology, for example.

OBJECTS Accordingly it is a principal object of the invention to providean improved image conversion device.

Another object of the invention is to provide an image conversion devicewhich produces improved sensitivity.

Another object of the invention is to provide an image conversion devicewhich permits direct or indirect interaction with the tube's targetplate by the incident radiation.

A further object of the invention is to provide an image conversiondevice which permits activation for neutron imaging.

Still a further object of the invention is to provide an imageconversion device which produces excellent contrast and resolution withsimplicity of circuitry.

For a complete understanding of the invention, together with otherobjects and advantages thereof, reference may be made to theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical representationof the open circuit voltage across a typical P-N junction as a functionof the light intensity incident on the P-N junction;

FIG. 2 represents a preferred embodiment of the invention wherein animage type target is formed for use in a device such as an orthicontelevision camera tube;

FIG. 3 shows diagrammatically part of the sectional view of the targetplate of the tube illustrated in FIG. 2; and,

FIGS. 4 and 5 represent alternative embodiments of the invention. I

DETAILED DESCRIPTION OF THE DRAWINGS The response of a typical P-Njunction utilized as a photovoltaic cell is shown in FIG. I. As can beseen the open circuit voltage of the P-N junction increases over a widerange proportionally with an increase in the intensity of the lightfalling upon the junction. This photovoltaic characteristic of P-Njunctions and its application to the invention will be more fullydescribed below.

The camera tube shown in FIG. 2 is a preferred embodiment constructedwith a target plate 10, the structure of which will be describedhereinafter with reference to FIG. 3. Electrical contact with the faceof the target plate 10 is made by use of an electrode 20 consisting of athin, semitransparent, and conductive layer, for example, of metal.Other means of electrical connection to this portion of the target platemay be employed, such as a ring contact. The target plate 10 with thesemitransparent electrode 20 is placed in the tube in such a manner thatit is opposite an electron gun aperture 30 of the type used in theconventional orthicon television camera, thereby replacing theconventional target plate. This return beam type of camera has a cathode40, an electron retarding element 50, and an amplifier 60.

The scope of this invention also includes tube configurations in whichthe signal is obtained at the target plate.

In a direct interaction arrangement as shown in FIG. 2, the source ofradiation can be, among others, visible light, X-rays, 'y-rays, B-rays,and a particles.

FIG. 3 is a sectional view of part of a typical P-N junction targetplate. The semitransparent electrode 20 is on the surface of the mosaicarray opposite the surface scanned by the electron beam. Each separateelement of the array consists of two regions of semiconductive materialof opposite conductivity type forming a junction. The bulk or backinglayer 70 is illustrated here as P type, and the other element of thejunction is illustrated as N type; however, this is a matter of choice.

Construction of the P-N junctions or other photovoltaic structures suchas P-l-N or surface barrier junctions can be accomplished in a varietyof ways, which are well known to those versed in the art. It is requiredthat the thickness of the radiation incident layer 70 be compatible withthe diffusion length of the minority carriers when used for opticalimaging or imaging with shallow penetrating radiations.

For purposes of illustration a silicon P-N junction is described but itis not intended by this to limit the scope of the patent to thismaterial or construction.

In operation, when a source of ionizing radiation 110 strikes the targetplate 10 there is released in the P zone 70 hole-electron pairs. In Ptype semiconductive material the electron is the minority carrier and itdrifts across the P zone until it reaches the dipole area of thedepletion region. At this point the electron is swept across thedepletion region to the N zone of the diode causing an excess ofelectrons in this zone providing a negative potential. When the targetplate 10 is scanned by the electron beam emanating from the electron gunaperture 30 this negative potential is sensed by the circuitry of thebeam, is amplified, and subsequently displayed on a video screen. Theobserved photovoltage developed at each junction is a function of localexcitation as shown in FIG. 1. In this way the pattern of the imagestriking the target plate can be obtained. For P regions on an N-typebase region the video signal is obtained from the current pulse derivedfrom the removal of radiation induced equilibrium charge.

By proper construction of the diode mosaic, the system should provideexcellent resolution (target grain size" of less than 0.001 in Themicrosecond response to image variation possible with the preferredembodiment is better than that obtainable with any of the prior artimage orthicon pickup tubes. With the orthicon tube a very small voltagevariation can be measured. For small amounts of radiation excitation thevoltage variation in the junction array is much greater than anycapacitance variation, thereby providing in a system employing thephotovoltaic characteristics of P-N junctions, better sensitivity thanthat obtained in systems employing only the capacitive characteristicsof P-N junctions.

FIG. 4 shows another embodiment of the invention for use with, forexample, ultraviolet and infrared sources. A photocathode 100 is placedbetween the diode mosaic 10 and the incident radiation 110. By applyinga uniform potential V4 the radiation produced photoelectrons areaccelerated to high energy by the electric field, and upon interactionof these electrons with the array, gain is produced. If, for example,the applied potential V4 is 3.6 Kev., a signal gain of about 10 can beobtained with a silicon target plate 10. However, since the efficiencyof the typical photocathode 100 is approximately 10 percent, a totalfinal gain of 10 will be obtained. A limitation of radiation sensitivityin this arrangement would be the photocathode dark current.

When the source of radiation is 'y rays or X-rays a further embodimentshown in FIG. 5 can be employed. A converter 120, for example, lead, isattached to the face of the target plate 10. The close proximity of thetwo surfaces is required because of the random dispersion of electronsin the converter 120 when struck by the radiation. These energeticelectrons then interact with the diode mosaic in the same manner asdescribed above.

This embodiment also applies to imaging with neutrons. In this instance,the converter layer is made up of a material having a high capture crosssection for neutrons (such as Li or B). The product particles theninteract with the diode array to produce the desired electron-holepairs.

The flux containing the image information of the object to be measuredmay be made incident upon either side of the target element, when theelectron beam scans the surface an improved collection efficiency isobserved because the minority carrier has less opportunity to recombinewith a minority carrier of the opposite polarity.

Although certain and specific embodiments have been illustrated, it isto be understood that modifications may be made without departing fromthe true spirit and scope of the invention.

I claim:

1. An image conversion system including a scanning electron beam, theimprovement comprising a target plate including a photovoltaic structureand electrical means of contact with said photovoltaic structure, saidtarget plate arranged in said system wherein said electron beam scansthe side of said target plate opposite said means of electrical contact,means for irradiating said target plate on the side of said electricalcontact, said irradiation exciting said photovoltaic structure producingan excess potential on said electron beam side of said target plate,said electron beam on scanning said target plate forming a closedelectrical path with said photovoltaic structure and said electricalcontact, and means for detecting the current variation in said closedelectrical path.

2. An image conversion system as set forth in claim 1 wherein saidphotovoltaic structure comprises a semiconductive material of a firstpolarity adjacent said electrical contact means and a plurality ofdiscrete semiconductors of a second polarity in contact with saidsemiconductive material of said first polarity, thereby forming P-Nsemiconductive junctions.

An image conversion system as set forth in claim 1 wherein saidphotovoltaic structure comprises a P-IN junction construction.

4. An image conversion system as set forth in claim 1 wherein saidphotovoltaic structure comprises a surface barrier-l-N junctionconstruction.

5. An image conversion system as set forth in claim 3 wherein said meansof irradiation is high energy X-rays.

6. An image conversion system as set forth in claim 3 wherein said meansof irradiation is ,8 rays.

7. An image conversion system as set forth in claim 3 where said meansof irradiation is a rays.

8. An image conversion system as set forth in claim 3 wherein said meansof irradiation is y rays.

9. An image conversion system as set forth in claim 3 wherein said meansof irradiation is high energy charged particles.

10. An image conversion system as set forth in claim 1 further includinga photocathode adjacent to said means of electrical contact, means forirradiating said photocathode, said photocathode producingphotoelectrons, and wherein said photoelectrons are given energy by anapplied field and which excite said photovoltaic structure.

11. An image conversion system as set forth in claim 10 wherein saidmeans of irradiation is ultraviolet light.

12. An image conversion system as set forth in claim 10 wherein saidmeans of irradiation is infrared light.

13. An image conversion system as set forth in claim 3 further includinga converter adjacent to said means of electrical contact andintermediate between said irradiating means and said photovoltaicstructure, said converter converting said irradiation of high energylevels to an energy level within the predetermined excitation range ofsaid photovoltaic structure.

14. An image conversion system as set forth in claim 13 wherein saidmeans of irradiation is 'y rays.

15. An image conversion system as set forth in claim 13 wherein saidmeans of irradiation is neutrons.

